1. Field
The present invention relates generally to the field of semiconductor manufacturing and more specifically to the formation of relaxed semiconductor buffer structures.
2. Description of the Related Art
Strained semiconductor materials advantageously provide improved electrical carrier mobility properties as compared to relaxed semiconductor materials, thus increasing the speed at which semiconductor circuits can operate. A semiconductor layer is said to be “strained” when it is constrained to have a lattice structure in at least one dimension that is the same as that of the underlying material's lattice structure, but different from the inherent lattice constant of the layer's material. Lattice strain occurs because the atoms in the deposited film depart from the positions normally occupied when the material is deposited over an underlying structure having a matching lattice structure. The degree of strain is related to several factors, including the thickness of the deposited layer and the degree of lattice mismatch between the deposited material and the underlying structure.
Strained semiconductor layers can be formed by epitaxially depositing silicon (Si) over a silicon-germanium (SiGe) buffer layer. Silicon-germanium films are used in a wide variety of semiconductor applications, such as in microelectronics fabrication. Because SiGe has a larger lattice constant than silicon, when epitaxial SiGe deposition occurs over silicon (such as during deposition on a silicon wafer), the epitaxially deposited SiGe is “strained” to the smaller underlying silicon lattice. If a strained silicon layer is to be deposited over the SiGe layer, the SiGe buffer layer should first be “relaxed” so that the silicon layer deposited thereover will be strained. In particular, because a strained SiGe layer has the dimensions of the underlying silicon lattice, a silicon layer deposited over a strained SiGe layer will not be strained. In contrast, a silicon layer deposited over a “relaxed” SiGe layer will be strained to conform to the larger underlying SiGe lattice. Thus, a strained silicon layer can be produced by epitaxially depositing silicon over a relaxed SiGe layer. There are a number of approaches to forming a relaxed SiGe layer over silicon.
In one approach, a SiGe layer is deposited beyond the “critical thickness.” As the thickness of a strained SiGe layer increases beyond a certain critical thickness, defects in the crystal structure of the strained SiGe layer appear, thereby inducing relaxation. After relaxation occurs, the degree of strain present in the SiGe layer is related to the amount of misfit dislocation generated in the layer during relaxation, which is a function of the elastic energy of the layer and the activation energy for dislocation nucleation and gliding. The critical thickness depends on a variety of factors, including growth rates, growth temperature, germanium concentration, and the number of defects within the layer underlying the SiGe layer. The SiGe can also be relaxed, for example due to macroscopic expansion, by annealing the workpiece after deposition.
Typically, a SiGe relaxed buffer layer is deposited with an increasing (or “graded”) concentration of germanium from the underlying silicon substrate to the top surface of the relaxed buffer layer. For example, the concentration of germanium can increase from about 0% to between about 20% and 40%. Grading the germanium concentration of the SiGe layer to gradually increase lattice constant with distance from the substrate facilitates relaxation and can minimize the generation of threading dislocations and pile-ups of threading dislocations (“pile-ups”).